1. Field of the Invention
The present invention relates to a circuit which is built in a communication terminal unit or the like for using a telephone line to perform communication and which generates an off-hook signal or dial pulses corresponding to a dial number.
2. Description of the Related Art
A conventional circuit for generating an off-hook signal and dial pulses has been disclosed, for example, in a reference material ("Type 600 Telephone Set", pp. 129, issued by Telecommunications Association (Jul. 15, 1964)). FIG. 53 is a block diagram showing a fundamental configuration of the conventional circuit 13 for an off-hook signal and dial pulses together with a telephone line (hereinafter also simply referred to as a line) 11 and a communication terminal transformer (hereinafter also simply referred to as a transformer). This circuit 13 is provided with a full-wave rectifying circuit 13a, a first switch block 13b connected in series with the + output terminal of the full-wave rectifying circuit 13a, a parallel circuit of a second switch block 13c and inductance for forming a loop current connected between the first switch block 13b and the - output terminal of the full-wave rectifying circuit 13a. In FIG. 53, C is a capacitor for cutting a direct current signal to a transformer L. In case of sending an off-hook signal (a line connection request signal) to a telephone office side from the circuit 13, first, a first control signal S1 is sent from a control circuit inside or outside a terminal unit to the first switch block 13b and the first switch block 13b is closed. Hereupon, an electric current limited by an internal resistance of the circuit 13 and a line resistance flows through the line 11, and this is transmitted to the office side as an off-hook signal. An off-hook signal reception response signal is returned from the unillustrated office side.
After confirming the off-hook signal reception response signal, a second control signal S2 is sent from the control circuit to the second switch block 13c, and the second switch block 13c is closed. Closing of the second switch block 13c protects the inductance 13d for forming a loop current and the transformer L so that a dial pulse signal which is a great-amplitude signal cannot flow through them. Subsequently, a signal which is turned off/on by the number of times corresponding to a dial number is sent from said control circuit to the first switch block 13b as a first control signal. Responding to this, dial pulses of intermittent line current are transmitted to the unillustrated office side. After the dial pulses have been transmitted, a control signal S2 is sent to the second switch block 13c from said control circuit and the control signal S2 opens the second switch block 13c and cancels said protected state. Thus the circuit 13 comes to be into a communicable state. After end of communication, a control signal S1 is sent from the control circuit to the switch block 13b and brings this circuit 13 into an open state, namely, a waiting state.
The inductance 13d for forming a loop current is provided for two purposes, as generally known. That is to say, the two purposes are roles of making an electric current of several tens milliamperes flow in order to let an office side recognize an off-hook state and of making no alternating current signal (no speech signal) flow in order to prevent occurrence of a speech loss caused by that a speech signal flows through said inductance during talking.
A waveform deterioration or a surge voltage is caused by letting a pulse current (concretely a dial pulse current) flow through this inductance 13d. Therefore, a configuration has been adopted in which a dial pulse current flows through the second switch block 13c by short-circuiting both ends of this inductance 13d with the second switch block 13c before generating the dial pulses. Accordingly, when generating the dial pulses, an electric current of the same in intensity results in flowing through the first switch block 13b and the second switch block 13c. Furthermore, the current flowing when generating dial pulses becomes sometimes about 80 mA or more, for example. Therefore, in a conventional circuit for generating an off-hook signal and dial pulses, there has been a first problem that the first and second switch blocks need to be respectively provided with switching devices (also called small power devices) capable of letting such a large current flow through them. The small power device is too large-sized to be built in a monolithic IC. Accordingly, it is necessary to externally attach such switching devices respectively to the first and the second switch block. Since this is not desirable in order to make a communication terminal smaller in size and higher in reliability, this is desired to be improved.
In a conventional circuit, due to a positional relation of arrangement of the switch blocks, connection/disconnection between a communication terminal transformer and a telephone line is performed in a short time by turning on/off the first switch block or the second switch block. Thereupon, there has been a second problem that there is a possibility that a surge voltage of a differential shape occurs at the time of said connection/disconnection. Since such a surge voltage may cause an erroneous communication response for example, it is desirable to be improved.